Oil resistant ethylene vinyl acetate elastomers (EVA) are well-known synthetic materials formed by copolymerizing ethylene and at least 40 wt % vinyl acetate. The ethylene vinyl acetate (EVA) copolymers may contain only copolymerized ethylene units and vinyl acetate units or the copolymers may comprise copolymerized ethylene units, vinyl acetate units, and additional copolymerized monomers, for example esters of unsaturated carboxylic acids, such as methyl acrylate or butyl acrylate. The raw polymers, also known as gums or gum rubbers, may be cured by free radical generators such as peroxides, azides, or by use of high energy radiation to form elastomeric articles. Examples of commercially available EVA copolymers that may be cured to form elastomeric articles include Elvax® resin products from E. I. du Pont de Nemours and Company and Levapren® products from Lanxess Corp.
In view of their low cost compared to other oil resistant elastomers, ethylene vinyl acetate copolymers are widely used in the manufacture of wire and cable jacketing as well as in the production of automotive parts such as hoses and seals.
Resistance to heat aging, meaning extended exposure to hot air, is a particularly desirable property in rubber parts that are used in under the hood automotive applications, e.g. hoses, gaskets, and seals. Because such parts may be exposed to temperatures in excess of 180° C. for periods of several hours on a regular basis, degradation of physical properties through oxidative embrittlement can occur. In articles made from cured ethylene vinyl acetate (EVA) articles, this often results in a reduction in extensibility and an increase in hardness and modulus of the rubber article. Such effects are disclosed for example in Patent Publication EP1081188. Methods to enhance heat aging resistance of elastomeric EVA articles have involved attempts to identify more effective antioxidant systems. However, there is still a need to improve the heat aging resistance of these copolymers.
Although it is known that the presence of fillers can have an adverse effect on high temperature stability of elastomers, the presence of fillers in elastomer formulations (also referred to in the art as elastomer compounds) is generally necessary for reinforcement and development of certain physical properties such as tensile strength and modulus in cured (i.e. crosslinked) compositions and in articles comprising the cured compositions. Carbon black is the most widely used filler due to its excellent reinforcement properties and low cost. Other examples of fillers that are commonly used for reinforcing EVA copolymers include hydrated alumina, calcium carbonate, barium sulfate, titanium dioxide, magnesium silicate, kaolin clay, and silica. All these fillers adversely affect heat aging of cured articles made from EVA copolymers.
It has been postulated that fillers accelerate heat aging of EVA copolymers by facilitating transport of oxygen to the polymer-filler interface. This leads to an increased rate of formation of free radicals at such locations through oxidative reactions. The free radicals generated in this manner promote crosslinking reactions, thereby resulting in eventual embrittlement of the elastomer. Reinforcing grades of carbon black such as N330 and N550 are particularly effective at facilitating transport of oxygen because they contain pores that may transport air. However, even non-porous fillers create interfacial regions between the solid filler particles and the elastomer. Few polymer chains reside in such interfacial regions and consequently diffusion of air may be enhanced. Thus, exposure of the elastomer to air is believed to be greater in filled EVA copolymers compared to EVA copolymers that are free of filler.
As the reinforcing power of a conventional filler increases, i.e., the ability of the filler to increase Shore A hardness of a cured elastomer composition, the tendency of that filler to lower resistance of the elastomer to the deleterious effects of heat aging also increases. It would be desirable to have available an alternative filler that permits the attainment of good elastic properties such as compression set resistance and tensile elongation to break in the cured, filled elastomer and further provides the advantages of filler reinforcement (i.e. high tensile strength, modulus and Shore A hardness), but does not promote oxidative degradation at high temperatures (i.e. 160° C. or greater).
It has now been found that it is possible to produce cured EVA elastomer compositions of high hardness, strength, and elasticity that exhibit excellent heat aging resistance through use of polyamide as a filler.
A number of EVA copolymer-polyamide blend compositions have been disclosed in the prior art. For example, it is known to add uncured EVA copolymers (i.e. gums) to polyamides to form toughened thermoplastic compositions. U.S. Pat. No. 4,174,358 exemplifies the use of uncured EVA copolymers at levels up to 20 wt % as toughening additives for polyamides. A compatibilizer such as a maleic anhydride grafted EVA copolymer may also be included in the EVA copolymer-polyamide blend, as disclosed in J. Polymer Science: Part B: Polymer Physics, Vol. 47, 877-887 (2009). The polyamide component in these compositions comprises the continuous polymer matrix and the uncured EVA copolymer is a minor additive. When polyamide comprises the continuous phase in the blend the composition generally cannot be processed at temperatures below the melting temperature of the polyamide, or can be processed only with great difficulty at such temperatures.
It is also known to form thermoplastic elastomer compositions comprising EVA copolymer and polyamide. For example, U.S. Pat. No. 5,948,503 discloses compositions comprising an uncured elastic polymer, a polyamide in the form of fine fibers, and a polyolefin having a melting temperature from 80° C. to 250° C. In addition, certain vulcanized compositions are disclosed therein.
Thermoplastic vulcanizates comprising EVA and polyamide, in which the EVA copolymer is dynamically crosslinked (i.e., crosslinked under shear mixing to create a dispersion of elastomer particles in a continuous phase of another polymer) are also known. Such compositions are disclosed in EP2098566, and may be improved by the use of a coupling agent such as maleic anhydride grafted EVA copolymer as disclosed in U.S. Pat. No. 7,691,943.
U.S. Pat. No. 7,608,216 and U.S. Patent Application Publication 2006/0100368 disclose compositions prepared by admixing an uncured elastomer, for example an EVA copolymer, with a thermoplastic polymer or another uncured (gum) elastomer. Techniques such as fractional curing, partial dynamic vulcanization, or the use of high performance reinforcing fillers are disclosed to increase the green strength of the uncured or partially cured compound. The admixed compositions may be subsequently crosslinked with a curing agent for the elastomer component.
As disclosed herein, it has now been found that when a dispersion of polyamide particles is used in place of all or a significant portion of a conventional particulate reinforcing agent in a continuous EVA elastomer, the resultant compositions, when cured by a free radical generator, exhibit enhanced resistance to physical property loss during heat aging. In addition, such compositions maintain excellent tensile strength, modulus, hardness, and elastic properties such as compression set and elongation at break that characterize compositions containing conventional reinforcing fillers.